The free radical polymerization of ethylenically unsaturated monomers is known. Polymers formed by this mechanism from monomers or oligomers having acrylic, methacrylic, vinyl ester and styrenic functionalities are major constituents in many films and coatings, including protective coatings, release coatings, adhesives and inks. Polymerization typically involves the use of an added compound--an "initiator"--that initiates the reaction of and chain formation by such monomers. When free-radical initiators are used, the initiation step consists of two reactions. In the first reaction, the initiator undergoes cleavage or dissociation upon exposure to a source of radiation (e.g., heat, ultraviolet light, etc.), causing the formation of a radical species of the initiator. In a second step, this radical then combines with a first monomer to form a chain initiating species of the polymer. Once formed, this chain initiating radical propagates the polymerization reaction, incorporating other monomers into a growing polymer chain.
When electromagnetic radiation is the source of energy used to initiate and polymerize free radically reactive monomers, initiators that absorb light and form radical species when exposed to energy in the ultraviolet to visible range (250 to 700 nm) are typically employed. These photoinitiators may be organic, organometallic, or inorganic compounds, but are most commonly organic in nature. Examples of commonly used organic free radical photoinitiators include benzoin and its derivatives, benzil ketals, acetophenone, acetophenone derivatives, and benzophenone derivatives.
While effective in the free radical polymerization of these monomers, the use of photoinitiators can often compromise the properties and purity of the polymerized material. Determining the optimal concentration of photoinitiator, particularly in thicker coatings, often requires making concessions between critical factors such as polymerization rate, curing at the surface or the bulk curing of the coating, and/or limiting the level of unreacted or residual monomers or photoinitiators. For example, lower photoinitiator levels tend to reduce residual photoinitiator content and allow the penetration of light through the depth of the coating, but also reduce the cure rate of the coating or film. Higher photoinitiator levels promote cure rate and surface cure of photopolymerized coatings, but potentially lead to incomplete polymerization of the coating's bulk and unacceptably high levels of residual photoinitiator. The presence of such residual photoinitiators and photoinitiator by-products is known to affect both the potential commercial applications and long term stability of photopolymerized coatings made in this manner.
Alternatively, electron beam radiation may be used to induce formation of radical species which can initiate chain growth and polymer formation. While electron beam cured coatings do not require addition of photoinitiators, several disadvantages of such coatings are well known. Cost to purchase and operate an electron beam is significantly greater than an ultraviolet source. In addition, electron beams are much less selective than ultraviolet light. Whereas light must be absorbed by a species for reaction to proceed, response of a material to an electron beam is only dependent on atomic number and a multitude of reaction pathways often are available. Further, depth of cure is limited by the specific energy of the electrons, usually restricting cure to depths of less than 0.005 dm. Substrate damage is also a concern in the use of electron beams because many common substrates are adversely affected by exposure to electrons.
Most commercial and research applications using free radically photopolymerizable monomers employ mercury vapor lamps to excite photoinitiators and propagate polymer chain growth, due to the relatively high efficiency, ease of operation, universal availability, and low cost of such lamps. Commonly available medium pressure mercury vapor lamps emit a broad spectrum of radiation across the ultraviolet and visible light ranges, and peak in intensity at emission ranges of 250 to 260 nanometers (nm) and 350 to 380 nm, depending on internal pressure within the bulb. Although formulations of photoinitiator and monomers generally are tailored to polymerize at these peak emissions, radiation at other wavelengths in this emission spectrum can result in undesired and deleterious properties in films and coatings polymerized using such mercury vapor lamps.
Recently, new ultraviolet light sources have become available which can deliver a monochromatic or narrow band output based upon excimer formation that occurs in certain noble gases or noble gas/halogen mixtures when exposed to high energy. The wavelengths of the emissions from these sources depend on the gases employed. For example, excimer sources containing xenon gas emit at a wavelength of 172 nm, xenon chloride excimer sources have a narrowband emission at 308 nm, and krypton chloride excimer sources generate ultraviolet radiation at 222 nm. Descriptions of the mechanisms by which these devices operate and the configurations of these devices are reviewed in Kitamura et al., Applied Surface Science, 79/80(1984), 507-513; German Patent Appl. DE 4,302,555 A1 (Turner et al.); and Kogelschatz et al., ABB Review, 3(1991), 21-28.
Excimer lamps have been used in the modification and microstructuring of polymer surfaces and the photodeposition of various coatings on metal, dielectric and semiconductor surfaces. Examples of these applications can be found in Kogelschatz, Applied Surface Science, 54(1992), 410-423, and Zhang et al., Journal of Adhesion Science and Technology, 8(10)(1994), 1179-1210.
European Patent Appl. EP 604738 A1 (Nohr et al.) describes a method of preparing a laminate which involves coating a cationically curable adhesive composition onto the surface of a first sheet, exposing the adhesive composition to ultraviolet radiation form an excimer lamp having a narrow wavelength band within the range of about 260 to about 360 nm, and bringing the surface of a second sheet in contact with the adhesive composition-bearing surface of the first sheet. The adhesive composition includes about 94 to about 60 percent by weight of a cycloaliphatic diepoxide, from about 1 to about 10 percent by weight of a cationic photoinitiator, and from about 5 to about 30 percent by weight of a vinyl chloride-vinyl acetate-vinyl alcohol terpolymer (based on the weight of the adhesive composition).
A process for producing coatings that improves upon mercury vapor lamp-based processes would be highly advantageous. It would be especially desirable to provide a free radical polymerization method for coatings that is initiator-free and which would thus yield coatings free of the residual initiator or initiator byproducts found in free radically polymerized materials prepared by other known methods.